1. Field of the Invention
This invention relates to the conversion of a hydrocarbon feed stream containing heavy olefins (4 carbon atoms per molecule, viz. C4, and heavier) to a product that is enhanced with an increased content of light olefins (ethylene and propylene, C2 and C3). In particular, this invention employs an oxygen containing co-feed material along with said feed stream to reduce the formation of aromatic compounds (C6 to C8) and heavier compounds (C8 and heavier) in the light olefin rich product of this invention. Hereinafter, all specified molecular carbon atom ranges, e.g., (C6 to C8), are inclusive.
2. Description of the Prior Art
The petrochemical industry has undergone sustained growth due, in part, to the demand for many, varied polymeric products. This has led to an ever increasing demand for petrochemical raw materials such as light olefins from which to make polymeric materials such as polyethylene and polypropylene.
Heretofore light olefins have been formed by the steam cracking of ethane, propane, or heavy hydrocarbons (C5 and heavier). Also, catalytic cracking of heavy hydrocarbons has been employed, for example, fluid catalytic cracking units. There is current interest in producing higher yields of light olefins from catalytic processes. See U.S. Pat. Nos. 5,043,522 and 5,026,936, both to Leyshon et al. In some of these processes the desired predominately light olefin product of the process contains undesirable quantities of aromatics (C6 to C8) and heavier hydrocarbons (C8 and heavier).
Also heretofore methanol has been catalytically converted to light olefins and butenes. See U.S. Pat. No. 4,590,320 to Sapre. Sapre teaches the use of a very minor amount, less than 3 weight percent, of a light olefin co-feed. This co-feed is expressly taught by Sapre to be used to avoid complications in tubular reactor stability due to autocatalysis. Thus, Sapre's minor amount of light olefin co-feed is taught not to take part in the chemical reaction that produces the light olefin product, but merely to serve as a mechanical, as opposed to chemical, means to thermally stabilize the reaction system.
It has also been taught heretofore to form olefins heavier than light olefins (C3 to C12) and/or paraffinic gasoline components (C4 and heavier) from methanol. See U.S. Pat. No. 5,191,142 to Marshall et al. Marshall et al. expressly teach that their invention is operable with a feed composed 100% of methanol, because Marshall et al. believe the mechanism of their invention to be methanol alkylation. Even though the presence of olefins is not required for the operability of the Marshall et al. invention, Marshall et al. teach that if some olefins do get into the feed due to the feed being the product of another process, e.g., a Fischer-Tropsch synthesis gas conversion process, then it is necessary for the operability of their invention that the methanol concentration “throughout the reaction zone” always be 50 to 100%, with a molar ratio of methanol to olefin of 2 to 1 being preferred. Thus, Marshall et al. teach the use of a substantial excess of methanol when an olefin is present.